The invention concerns a method for producing a foamed slag in a metal bath in a metallurgical furnace, in which a mixture that contains at least a metal oxide and carbon is introduced into the furnace, where below the slag present in the furnace, the metal oxide is reduced by the carbon, where the gases formed during the reduction form bubbles in the slag, thereby foaming the slag, and where the mixture is introduced into the furnace in such a way that a desired height or a desired range of heights of the layer of foamed slag develops and is maintained.
A method of this general type is disclosed by EP 0 637 634 A1. This previously known method can be used to produce a foamed slag on a metal bath, for example, on a bath composed of stainless metal. During the melting of the solid metal in an electric arc furnace, a slag forms that can contain a large fraction of Cr oxide. The concentration of this fraction often reaches values of greater than 30%. Due to their composition, slags of this type cannot be liquefied and foamed to the desired extent with the prior-art method. Similar solutions are disclosed by DE 26 08 320 B1, WO 01/72090 A1, WO 99/23264, and WO 2004/104232 A1.
The cited document describes the addition to the metal bath of a mixture that contains at least a metal oxide and carbon. In addition, the mixture can contain an iron carrier material and a binder. The mixture can be added to the bath compressed and in the form of pellets or briquettes. If the mixture is introduced into the region between the metal bath and the slag layer, it can chemically react there, and a metal oxide reduction process starts to occur. This process of reduction of the metal oxide with the carbon leads to the formation of gaseous carbon monoxide (CO), which results in bubble formation, which produces foaming of the slag.
The advantage of producing a foamed slag consists in the following: During the operation of an arc furnace, the charge, e.g., the scrap to be melted down, is melted in the furnace by means of the arc of the electrodes. In this connection, the slag fulfills not only its primary function of removing undesired constituents from the metal bath but also a protective function due to its foamed state. Specifically, the foamed slag at least partially fills the space between the ends of the electrodes and the surface of the molten metal and thus protects the refractory lining of the furnace from radiant energy of the electric arc.
Due to the low thermal conductivity of the foamed slag, the radiation of the electric arc towards the wall of the arc furnace is greatly reduced, and thus the energy input into the metal bath is improved.
Another advantage of the foamed slag is its noise muffling effect. The covered or enveloped electric arc thus emits less noise into the surroundings, which improves the environmental conditions in the vicinity of the furnace.
Although it is possible to produce a foamed slag with the prior-art method, a disadvantage is that exact control of the amount of foamed slag is difficult.